1. Field of the Invention
The present invention relates in general to the field of digital telecommunications, and more particularly to a method and apparatus for monitoring and testing the performance of circuits used to transmit digital information between a telecommunications network and a network user.
2. Description of Related Art
For some time, public switched telephone networks (PSTN) have utilized time division multiplexing (TDM) transmission systems to communicate both voice and data signals over a digital communications link. For example, DS1 data paths have long been used to carry both voice and data signals over a single transmission facility. DS1 data paths carry DS1 (digital signal level 1) signals which are transmitted at a nominal rate of 1.544 Mb/s. As is well known, DS1 data paths advantageously reduce the number of lines required to carry information that otherwise would be required without time division multiplexing the voice and data signals.
While DS1 facilities are used in large part to carry signals switched by components of the PSTN, point-to-point DS1 data links are also used to connect the premises of data users to one another. A typical application is shown in FIG. 1. Much of the growth in the network in recent years have been in such high capacity digital service (HCDS) applications.
DS1 transmission systems, like the one shown in FIG. 1, include three general equipment types: terminating equipment 10, user interface equipment 20, and transmission equipment 30. Terminating equipment 10 primarily serves to build the DS1 1.544 Mb/s TDM signal from the various sub-rate voice and data signals. Terminating equipment 10 typically performs pulse code modulation (PCM) and TDM functions. The terminating equipment 10 also de-multiplexes the 1.544 Mb/s DS1 signal to separate voice and data signals at their original sub-rates.
DS1 signals are typically transmitted over a dedicated point-to-point network as simple as the one shown in FIG. 1 utilizing twisted wire pairs and repeaters spaced at intermediate points or as complex as the one shown in FIG. 2 which may utilize any combination of twisted wire pairs and repeaters, multiplexers, Digital Cross-connect Systems (DCS), Add Drop Multiplexer (ADM), Fiber Optic Terminals (FOT), Coaxial Cable, Microwave, Satellite, or any other transmission media capable of transporting a DS1 signal. In some instances, the DS1 may be carried over a network similar to the point-to-point network with the added capability to switch the DS1 signal (in a DCS or ADM) in a manner similar to the PSTN.
The user interface equipment 20 connects the terminating equipment 10 with the transmission equipment 30, such as a DS1 data path and ensures that both of the ends of the DS1 data paths 30 send and receive a high quality DS1 signal. As such, the user interface equipment 20 typically checks for conformance to certain standards which are set by the telecommunications industry. The user interface equipment 20 corrects and detects errors in the DS1 transmission path. For example, the user interface equipment 20 corrects for Bipolar Violations (BPV). In addition, the user interface equipment 20 detects and inserts various errors, alarms, and zero substitution codes in the DS1 transmission path, including yellow alarms, alarm indication signals (AIS), and Bipolar with Eight-Zero Substitution (B8ZS) signals.
The DS1 data path 30 is the hardware used by the network providers to transmit the DS1 digital signal over a distance. The DS1 data path 30 as shown is implemented by a T1 line. However, coaxial, fiber optic cables, and microwave links may be used by providing an appropriate transport interface between the CSU 107 and the facility. Twisted pair cable used to catty DS1 signals require repeaters at approximately 7000 foot intervals.
While DS1 transmission systems such as the system shown in FIG. 1 are well-known, customers more typically communicate using a public DS1 network, as shown in FIG. 2. In the DS1 transmission system shown in FIG. 2, equipment is divided into categories based on the location of the equipment. Essentially, the equipment is broken into three categories: (1) the customer premises equipment (CPE) 40; (2) the local loop equipment 42; and (3) the central office (CO) equipment 44. CPE 40 belongs to the network user. The organization or customer that owns the CPE 40 is responsible for both its operation and maintenance. As the responsible party, the customer must ensure that its equipment provides a healthy and standard DS1 digital signal to the public local loop equipment 42. The equipment 40 on the customer premises typically consists of DS1 multiplexers 46, Digital PBXs, or any other DS1 terminating equipment which connects to the channel service units (CSU) 48. The local loop equipment 42 essentially serves to connect the CPE 40 with the central office 44. Local exchange carriers (LECs) assume responsibility for maintaining equipment at the line of demarcation between the CPE 40 and the local loop 42.
As shown in FIG. 2, a network interface unit (NIU) 50 couples the CPE 40 with the local loop 42. The NIU 50 represents the point of demarcation between the CPE 40 and the network equipment (which comprises local loop equipment 42, COs 44, and a DS1 network 52). Prior art NIUs are relatively simple devices which allow network technicians to minimally test the operation and performance of both the CPE 40 and the DS1 network 52.
Central Office equipment 44 connects the DS1 signals customers and routes the traffic through the DS1 network 52 based upon a final destination provided by the CPE 40. The Central Office equipment 44 can serve as a test access point for various DS1 signal requirements.
Independent of whether the DS1 transmission system is simple (FIG. 1), complex (FIG. 2), or switched, all the circuits and network equipment required to transmit a DS1 signal must be properly tested and maintained to operate at maximum efficiency. All DS1 testing falls into one of two categories: (1) out-of-service testing, and (2) in-service monitoring. Out-of-service testing causes live traffic to be removed from the DS1 link before testing commences. In out-of-service testing, a test instrument transmits a specific data pattern to a receiving test instrument that anticipates the sequence of the pattem being sent. Any deviation from the anticipated pattern is counted as an error by the receiving test instrument. Out-of-service testing can be conducted on a "point-to-point" basis or by creating a "loop-back". Point-to-point testing requires two test instruments (one instrument at one end of the DS1 transmission system, and one instrument at the other end of the DS1 transmission system). By simultaneously generating a test data pattern and analyzing the received data for errors, the test instruments can analyze the performance of a DS1 link in both directions.
Loop-back testing is often used as a "quick check" of circuit performance or when attattempting to isolate faulty equipment. In loop-back testing, a single test instrument sends a "loop-up" code to a loop back device, such as the CSU at the far end, before data is actually transmitted. The loop-up code causes all transmitted data to be looped back by the CSU in the direction toward the test instrument. By analyzing the received data for errors, the test instrument measures the performance of the link up to and including the far end CSU. Because loop-back testing only requires a single test instrument, and thus, only one operator, it is a convenient testing means.
Both point-to-point and loop-back tests allow detailed measurements of any DS1 transmission system. However, because both testing methods require that live, revenue-generating traffic be interrupted, they are impractical. Thus, out-of-service testing is inherently expensive and impractical. It is therefore desirable to perform in-service monitoring of "live" data to measure the performance and viability of DS1 transmission systems. Because in-service monitoring does not disrupt the transmission of live, revenue-generating traffic, it is suitable for routine maintenance and it is preferred by both the LECs and their customers.
Referring again to FIG. 2, the prior art NIUs 50 disadvantageously provide only intrusive test and performance monitoring functionality. End-user customers object to the service interruptions and disruptions required by the out-of-service testing performed by the prior art NIUs 50. The LECs install the NIUs 50 at the demarcation point between the CPE 40 and the LEC portions of the network (i.e., at the interface to the local loop 42). The prior art NIUs 50 typically have provided the LECs with a loop-back point for testing DS1 digital circuits to the network boundary. Disadvantageously, customer circuits may be taken out-of-service for intrusive testing only with customer permission. Customers typically do not authorize such intrusive testing means unless a circuit is completely unusable.
There are several types of NIUs 50 currently in use. For example, one of the most popular types of NIUs 50 is the "Smart Jack" available from Westell, Inc., located in Oakbrook, Ill. The Smart Jack NIU with Performance Monitor (PM) allows the LECs to determine what errors are received and generated by the CPE 40. A major disadvantage of the Smart Jack NIU is that the NIU accumulates PM data and stores the data in a local buffer for later retrieval by LEC personnel. Data retrieval in most areas requires that a circuit be taken completely out-of-service and that the NIU be commanded intrusively using a proprietary command set. Furthermore, the Smart Jack NIU disadvantageously provides no practical method for the LECs to retrieve the performance monitor data collected by the Smart Jack NIU. While the Smart Jack NIU does allow non-intrusive transmission of PM data from the Smart Jack to the central office, a paralleling maintenance line must be provided. Most DS1 installations to customer premises are, however, unprotected by such maintenance lines.
Other NIUs 50 are available from Wescom Integrated Network Systems (WINS), the Larus Corporation, and Teltrend, Inc., a wholly owned subsidiary of T1 Holdings, Inc., located in New York, N.Y. All of the prior art NIUs 50 suffer the disadvantages associated with out-of-service monitoring and testing. Therefore, there is a need for an improved NIU 50 which provides non-intrusive maintenance performance monitoring at the point of demarcation between the CPE 40 and the LEC equipment. In addition to being unable to provide non-intrusive testing and monitoring of DS1 digital equipment, the prior art NIUs 50 are unable to provide an indication of a loss of signal (LOS) caused by the CPE 40 which is distinguishable from LOSs that are caused by failure of the network equipment. Currently, LOS caused by the CPE 40 generates alarms in the LEC central office equipment 44 which are indistinguishable from the alarms generated in response to LOSs caused by equipment failures in the local loop 42, Central Office 44, or DS1 network 52. Therefore, there is a need for an enhanced NIU which allows the LECs to detect LOS alarm signals that originate within the CPE 40. With such an enhanced NIU, the LECs can then decide whether to notify their customers of the LOS indication or to ignore the indication as they deem appropriate.
In addition to these disadvantages, the prior art NIUs 50 do not permit the LECs to control the frame format of data transmitted by their customers and transmitted over the LECs' network. In general, DS1 signals can be transmitted to the local loop 42 using four basic DS1 frame formats: (1) Super Frame format (SF); (2) Extended Superflame Format (ESF) without Performance Report Messages (PRMs); (3) ESF with AT&T PUB 54016 Performance Reporting; and (4) ESF with ANSI T1.403 Performance Report Messages (PRMs). Most DS1 signals are transmitted using the SF format, and the remainder are transmitted by the CPE 40 using a mix of ESF format types. Performance monitoring capabilities of the various formats range from poor in the case of SF (most of the data is not monitored), to excellent, in the case of ESF with ANSI T1.403 PRMs. The difficulty faced by the LECs is that their ability to monitor data and transmission performance is tied to the frame format used by the CPE 40. Because the customer is responsible for the CPE 40, the LECs are unable to control the frame format used and thus the level and extent of performance monitoring and testing that is achievable. The present invention allows the LECs to control the frame format of data transmitted by the CPE 40.
The ESF format has long been recognized as the single most important change occurring in the telephone network with respect to the quality of service provided on DS1 circuits because it addresses the above-stated need for non-intrusive monitor and test capability. ESF allows customers to continuously and non-intrusively monitor the performance of their DS1 facilities while the applications remain active and thus income-generating. ESF performance monitoring provides both a precise performance report and a proactive maintenance tool. With ESF performance data, a customer can determine correlations between data application performance (response time) and errors which occur on the DS1 facilities. This can aid in troubleshooting end-user response time problems. By looking at the error conditions, the cause of the increased response time can be determined and the appropriate action can be taken.
In addition, the ESF frame format offers the network providers the ability to "sectionalize" problems occurring in the network. By placing ESF monitoring equipment throughout the network, an LEC can monitor the various facilities that make up an end-to-end customer circuit. When customers complain about a degraded or unavailable circuit, the LEC can use the ESF format to locate the faulty link in a real time, non-intrusive manner.
Although the ESF frame format has long been recognized as a tremendous benefit, it has gained little acceptance and use in the CPE 40. Therefore, there is a need for an improved NIU which allows telephone companies to add the ESF functionality to existing DS1 circuits. There is also a need for an enhanced NIU which will provide telephone companies an adaptive way to increase the number of circuits that use the preferred ESF signal format as the circuit enters the LEC equipment. Moreover, there is a need to combine the functions of network interface, circuit loop-back, frame format conversion, and CPE loss of signal detection functionality together in an inexpensive and easily accessible NIU. The present invention provides such an improved NIU.